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Background  In the fungal pathogen  Candida albicans,  amino acid substitutions of 14alpha-demethylase (CaErg11p, CaCYP51) are associated with azole antifungals resistance. This is an area of research which is very dynamic, since the stakes concern the screening of new antifungals which circumvent resistance. The impact of amino acid substitutions on azole interaction has been postulated by homology modeling in comparison to the crystal structure of  Mycobacterium tuberculosis  (MT-CYP51). Modeling of amino acid residues situated between positions 428 to 459 remains difficult to explain to date, because they are in a major insertion loop specifically present in fungal species.    Methodology/Principal Finding  Fluconazole resistance of clinical isolates displaying Y447H and V456I novel CaErg11p substitutions confirmed  in vivo  in a murine model of disseminated candidiasis. Y447H and V456I implication into fluconazole resistance was then studied by site-directed mutagenesis of wild-type CaErg11p and by heterogeneously expression into the  Pichia pastoris  model. CLSI modified tests showed that V447H and V456I are responsible for an 8-fold increase in fluconazole MICs of  P. pastoris  mutants compared to the wild-type controls. Moreover, mutants showed a sustained capacity for producing ergosterol, even in the presence of fluconazole. Based on these biological results, we are the first to propose a hybrid homology structure-function model of Ca-CYP51 using 3 different homology modeling programs. The variable position of the protein insertion loop, using different liganded or non-liganded templates of recently solved CYP51 structures, suggests its inherent flexibility. Mapping of recognized azole-resistant substitutions indicated that the flexibility of this region is probably enhanced by the relatively high glycine content of the consensus.    Conclusions/Significance  The results highlight the potential role of the insertion loop in azole resistance in the human pathogen  C. albicans . This new data should be taken into consideration for future studies aimed at designing new antifungal agents, which circumvent azole resistance.

Amino Acid Substitutions at the Major Insertion Loop of Candida albicans Sterol 14alpha-Demethylase Are Involved in Fluconazole Resistance